#include <stdio.h>
#include <string.h>
void reverse(char *p) {
  int len;
  char c, *q;
  len = strlen(p) - 1;
  for (q = p + len; p < q; p++, q--) {
    c = *p;
    *p = *q;
    *q = c;
  }
}
int main() {
  char str[] = "This is a pen.";
  puts(str);
  reverse(str);
  puts(str);
  return 0;
}
/* end */

This is a pen.
.nep a si sihT